In wafer level packaging applications, solder is applied directly to wafers that have completed the fabrication of microelectronic devices, but have not been diced into individual chips. A polymeric photoresist is typically applied to a substrate as a mask to define the placement of solder onto wafers. After solder is deposited onto the wafer, the photoresist must be removed before the next step in the packaging process can occur. In some cases, a positive photoresist may be used. Exposure of the positive photoresist to actinic radiation may cause a chemical reaction resulting in a solubility increase in aqueous alkali that allows the positive photoresist to be dissolved and rinsed away with developer. In other cases, a negative photoresist may be used. When the negative photoresist is exposed to actinic radiation, cross-linking of the polymer may occur in the exposed regions while leaving unexposed regions unchanged. The unexposed regions may be subject to dissolution and rinsing by a suitable developer chemistry.
Due to the thickness and cross-linked nature of thick dry film negative photoresist, the removal of this material after solder deposition can be difficult. The typical process for removing thick dry film negative photoresist in wafer level packaging applications is immersion of the wafer in formulated organic solvent-based mixtures for extended periods of time, often longer than 1 hr. Typically, 25 wafers are immersed in a tank containing the formulated solvent-based mixture for a sufficient time to completely remove the photoresist film. After a sufficient period of time, the wafers are transferred to additional tanks for rinsing, where the rinsing media may include water or isopropanol. Additional wafers are then processed in the same tank reusing the same formulated mixture, and the process is repeated for as long as the formulated mixture is capable of sufficiently removing the photoresist completely from the wafer.
Often, immersion-based removal of photoresist results in Sn being extracted from the solder and causing a thin film containing Sn to be plated onto field metal on the substrate. In some cases, the Sn-containing film may cover at least a portion of the field metal. If the Sn-containing film is not removed, the field metal cannot be removed, which may cause device failure. The Sn-containing film is often removed from the field metal by a plasma-based etch process, which requires the use of an additional process step on an additional piece of equipment in between the photoresist removal step and the field metal etch step.